Method for the production of electrodes on a type II or VI semiconductor material or on a compound of said material

ABSTRACT

A method for manufacturing electrodes on a semiconducting material of type II-VI or on a compound of this material. The electrodes are preferably in gold or platinum and are formed by electrochemical deposition of gold or platinum from a solution of gold or platinum chloride in pure hydrochloric acid.

TECHNICAL FIELD

The present invention relates to a method for manufacturing electrodeson a semiconducting material of type II-VI or on a compound of thismaterial.

It particularly applies to X or gamma ray detectors and moreparticularly

-   -   to blocks of individual detectors, capable of being juxtaposed        in a detection array, and    -   to monolithic detectors including multiple electrodes on one        face (pixellization) and a biasing electrode on another face.

The invention in particular relates to the manufacturing of electrodeson detectors made from semiconducting materials of type II-VI such asCdZnTe, CdTe, CdTe:Cl, CdTeSe:Cl, CdZnTe;Cl, CdTe:In, CdZnTe:In andCdHgTe for example.

These tellurium- and cadmium-based semiconductors may be obtained with aBridgman or THM type growth method for example and as a major feature,they have very high electrical resistivity (larger than 10⁹ Ω·cm) whichis absolutely necessary for making X or gamma spectrometry devicesdedicated to medical, industrial or scientific imaging.

The method, subject of the invention, may directly be used industriallybecause of its simplicity, its suitability for present products of thesemiconductor industry (positive resists) and of its moderate price. Itis particularly suitable for X or gamma detectors but is not limited tothis application.

STATE OF THE PRIOR ART

Different types of radiation detectors are known, among which appear gasdetectors, scintillator detectors, and semiconductor detectors. Thelatter have the advantage of having a large atomic number allowing amaximum number of incident photons to be absorbed for a minimum materialthickness.

These semiconductor detectors generally are of a parallelepipedous shapeand are cut out in an ingot slice, including at least two parallel faceson which electrical contacts are made for biasing the detectors and forcollecting electrical signals produced by the incident radiations.

The electrical contacts should not notably change the behavior of thecorresponding detector and therefore should have insignificantresistance to current, as compared with that of the material of thedetector.

They should therefore be of the ohmic type, i.e., have a quasi-linearcurrent-voltage characteristic and utilize a judicious band curvature atthe metal-semiconductor contact, a tunnel effect at this location andrecombinations in the space charge area.

Manufacturing of such ohmic contacts on the aforementioned materials(II-VI) remains a delicate problem to be solved, because, in addition tothe suitable electrical behavior which should be obtained, theseelectrodes need to be connected to a read-out circuit, for example.

The connection should by no means change the electrical behavior of thecontact. It generally induces stresses which the contact should be ableto accept. Its adhesion is therefore essential all the more so as thematerials in presence do not necessarily have the same expansioncoefficients. Temperature variations therefore induce furtherconstraints which may be very large. From now on, let us specify thatthe invention may be applied with compounds based on Cd and Te, of typep or type n.

In the case of CdTe and compounds of the latter having high resistivity,it is difficult to elaborate an ohmic contact because most metals have awork function less than that of CdTe, which is equal to 5.02 eV.

Only platinum (5.3 eV) and gold (4.8 eV) approach this value. Contactsmade by vaporization or sputtering of both of these metals are neitherohmic nor blocking but are located between both of them. Otherphysico-chemical phenomena, such as surface states before depositing themetal or oxidizing the surface, set the height of the potential barrierregardless of the metal's work function.

It is possible to obtain an ohmic contact if the charge carriers mayfreely migrate by the tunnel effect. This transport mode is favored byelectrochemical deposition of solutions of the gold chloride (AuCl₃) orplatinum chloride (PtCl₄) type on a surface chemically etchedbeforehand. The metal is chemically reduced by tellurium and acts as astrong acceptor dopant at the surface of the detector. On this subject,reference will be made to the following document:

[1] J. P. Ponpon, Solid-state Electronics, Vol. 28, No.7, pp. 689-706,1985.

As is seen, gold and platinum are among the best candidates for makinghigh resistivity ohmic contacts on CdTe and on compounds of the latter.Their deposition is preferably achieved through an electrochemical route(to promote the tunnel effect), on a suitably prepared surfacebeforehand.

On this matter, reference will be made to the following document:

[2] E. Janick et al., J. Phys. D: Appl. Phys., 16 (1983), pp. 2333-2340.

Let us specify that CdTe or the compounds of the latter are used in thepolycrystalline or monocrystalline form and are of type n or p.

Further, the structure of these materials induces a polarity:stoichiometry is different according to the relevant face. It isbelieved that this polarity induces specific mechanical and chemicalbehavior according to the relevant case.

Setting the detector to the desired dimensions is generally accomplishedby cutting, grinding then polishing, which allows surfaces to beobtained, for which the perturbed thickness is minimal and may beremoved by chemical etching without changing the flatness and the finaldimensions too much, because this thickness may contain interferingcontaminants from the electrical point of view and increase theresistance at the interface between the material of the detector and anelectrical contact subsequently formed on the latter.

Two types of solutions are generally used for chemically preparing thesurface:

-   -   acid solutions based on potassium bichromate or a mixture of        different acids such as HF, HNO₃, H₂SO₄, CH₃COOH;    -   mixed acid/solvent solutions such as the Br-methanol solution,        mostly used in the case of CdTe or a compound of the latter.

A large number of solutions are known. Their effect on surfacestoichiometry may differ from one solution to another, in particular asto the adhesion of a metal layer, intended to form an electricalcontact, and as to the electrical behavior of the detector, aftercompletion of the latter.

Gold and platinum are well-suited for forming electrodes, in particularin the case of CdTe and of compounds of the latter. Deposited via anelectrochemical (electroless) route, from a chloride of the relevantmetal (gold or platinum), this metal takes the place of cadmium on thesurface of the detector. As for cadmium, it enriches the solution usedfor depositing the metal.

This solution generally consists of gold or platinum chloride and wateror optionally a solvent, such as for example ethylene glycol.

The maximum obtained thickness depends on the chloride concentration inthe solution but it is generally less than 150 nm for Au and Pt, becauseof the polarization effect which is induced by the reaction fordepositing the metal (Au or Pt).

The contacts formed with gold or platinum by electrochemical(electroless) coating on CeTe or on CdTe compounds should meet

-   -   electrical specifications because there must be a low resistance        at the metal/semiconductor interface in order to be able to        collect the maximum number of charges from the detector;    -   mechanical specifications because there should be sufficient        contact thickness, so that subsequent connection to another        component does not deteriorate the underlying material or the        characteristics of the latter, and sufficient adhesion in order        to accept mechanical stresses due to the different expansion        coefficients.

Indeed, since conductivity of CdTe is very low, any variation oftemperature induces stresses in the electrical contact.

Further, in electrochemical (electroless) deposition itself, stressesincrease with the thickness of the layer, which forces this thickness tobe limited to 50 nm, hence risks of deteriorating themetal/semiconductor interface at the moment of the connection.

Stresses are also induced by a further possible deposit on the contact(for example a deposit of conducting adhesive) or by soldering (with adifferent material for example, indium for collective connections of theflip chip type and therefore an increase in temperature (about 160° C.for indium) with creation of an alloy.

Further, stresses are induced on the contact by assembly with aconnection circuit or a read-out circuit in a casing.

Adhesion of the contact therefore depends on the surface preparation, onchemical reactions at the metal/semiconductor interface, on thethickness of the coating and also on the surface of the latter.

Now, the present trend in the nuclear detection field for example, is tomanufacture pixellized monolithic detectors or juxtapose unit detectors.The surface of the contacts may then decrease from 5 mm² to 50 μm².Adhesion then becomes very critical.

Adhesion of the contacts is presently considered to be an importantproblem. On this matter, reference will be made to the followingdocument:

[3] V. Gostilo et al., Nucl. Instr. and Meth. in Phys. Res. A 460(2001), pp. 27-34.

Indeed, a gold layer deposited from an aqueous solution (respectivelyfrom a solution in a solvent such as ethylene glycol) withstands apeeling force not exceeding 1 kg/cm² (respectively 2 kg/cm²) for a layerwith a thickness less than 50 nm. For larger thicknesses, adhesion isstrongly reduced.

As for platinum, when it is deposited from an aqueous solution at 30°C., adhesion is a little better but strongly drops when the thicknessexceeds 100 nm.

DISCUSSION OF THE INVENTION

The object of the method, subject of the invention, is to form on a CdTetype material or on compounds of this material, electrodes which havegood electric properties but especially a large thickness and adheringcapacity. To do this, one proceeds with electrochemical (electroless)deposition of gold or platinum chloride in solution with purehydrochloric acid.

Advantageously, preparing the surface of the material used (CdTe or acompound of CdTe) is performed from a solution of Br and preferably purehydrochloric acid.

More generally, the present invention solves the problem ofmanufacturing electrodes on a semiconducting material of type II-VI, oron a compound of this material, these electrodes being able to havelarger thickness and better adhesion onto the material than in the priorart.

Specifically, the object of the present invention is a method formanufacturing at least one electrode on a II-VI semiconducting materialor a compound of this material, this electrode being in metal for whichthe work function is substantially equal to or larger than that of theII-VI semiconductor, this method being characterized in that theelectrode is formed by electrochemical deposition of the metal from asolution of a chloride of the metal in pure hydrochloric acid.

Preferably, the metal is gold or platinum, and a gold or platinumchloride solution in pure hydrochloric acid is used.

Further, preferably, the concentration of gold or platinum chloride inthe pure hydrochloric acid is less than 5%.

According to a preferred embodiment of the invention, the surface of thematerial is prepared before the deposition in order to make this surfacecapable of fixing the metal.

To do this, the material surface may be chemically etched.

In this case, according to a particular embodiment, the metal being goldor platinum, a gold or platinum chloride solution in pure hydrochloricacid is used and a solution of bromine and preferably pure hydrochloricacid, is used for the chemical etching.

Preferably, the material is CdTe. In this case, the electrode may forexample be formed on a material which is selected from CdZnTe, CdTe:Cl,CdTeSe:Cl, CdZnTe:Cl, CdTe:In, CdZnTe:In and CdHgTe.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

With the invention, gold or platinum layers may be made, for which thethickness is between 100 nm and 150 nm and which withstand peelingforces larger than 10 kg/cm², by a suitable chemical preparation of thesurface of CdTe or of a compound of the latter, followed byelectrochemical (electroless) deposition of the metal from an acidsolution.

More specifically, chemical preparation of the surface consists ofremoving the thickness damaged by the machining (grinding, polishing)and creating a surface to which a subsequent deposition is able toadhere and which is chemically favorable for obtaining a low resistancemetal/semiconductor interface and this preparation is performed from asolution consisting of Br and preferably pure hydrochloric acid in asuitable proportion (a few % of Br).

The dissolution of Br in preferably pure hydrochloric acid is very slowand requires stirring for several minutes.

After chemical etching, the sample is rinsed (for example twice) inpreferably pure hydrochloric acid then dried directly. It is notrecommended to put it in presence of water.

The properly dried sample is then put into contact with theelectrochemical (electroless) solution, the essential feature of whichis that gold or platinum chloride is dissolved in pure hydrochloricacid.

The chloride concentration is preferably low (less than 1% in the caseof gold chloride) because deposition kinetics is important for the finalresult.

When deposition is completed, rinsing is performed in hydrochloric acidthen in water, followed by suitable drying.

The whole of this method is applied in an acid medium, which leads toprotecting the portions of the sample which must not undergo chemicaletching or receive the metal coating, with positive photoresists whichare conventionally used in the semiconductor industry.

These positive resists are soluble in standard solvents such as methanolfor example. Now, the chemical preparation of a surface is very oftencarried out with a Br-methanol mixture.

Furthermore, deposition of gold is also sometimes performed in presenceof a solvent.

Further, the surface tension of both solutions (one for the preparationand the other for the deposition) is such that it allows the use ofdrops of these solutions and therefore chemical etches and coatingswhich are limited to the surfaces intended for the contact. Theseoperations are unachievable with the Br-methanol mixture or withchloride solutions in a solvent, such as ethylene glycol for example.

Let us also recall that chlorine is used as dopant in semiconductingmaterials, the chlorine containing medium being thereby favorable tosuch materials.

Comparative tests performed on the same material (CdTe) but withdifferent contact technologies, show that the detection efficiency ofdetectors made with this method is at least equal to that of detectorsmade with conventional technologies.

Further, the useful life of the Br solution in hydrochloric acid is long(several days) whereas the conventional Br-methanol solutions onlyretain their aggressivity for a few hours because of evaporation of thebromine. In addition, the concentration for example of gold in thecoating solution, is much less than that of conventional aqueoussolutions (1 g for 30 cm³ of water), which is of interest from theindustrial point of view.

This method was applied on CdTe (HPBM) or CdTe:Cl (type p) unitdetectors and also on pixellized monolithic detectors. Their electricalbehavior shows substantial improvement and very high adhesion from themechanical point of view.

Another example of the method, subject of the invention, is givenhereafter, for making gold electrodes on a CdTe substrate.

To do this, a solution of gold chloride in pure hydrochloric acid isused, the concentration of gold chloride in this acid being less than1%.

One then proceeds with electrochemical deposition of gold on the areasof the substrate where the intention is to form electrodes, by puttingthe solution in contact with these areas.

The portions of the substrate which are not intended to be put intocontact with the solution, in particular the portions on which gold isnot intended to be deposited, are protected beforehand. To do this, aprotective coating is formed on these portions and the coating isremoved after having deposited gold at the desired locations.

As a protective coating, a photoresist layer may be used for example.

In addition, it is preferable to prepare the surface of the substratebefore depositing gold thereon, notably to make this surface capable offixing gold.

To do this, one may proceed with chemical etching of this surface bymeans of a bromine solution in preferably pure hydrochloric acid.Preferably, the thereby etched surface is rinsed by means of the sameacid then by water, and then it is dried.

As seen earlier, platinum may be used instead of gold.

Instead of a CdTe substrate, a substrate made out of a compound of CdTemay be used according to the invention for forming electrodes thereon.Examples of such a compound have already been given above.

It is recalled hereafter what is meant by pure hydrochloric acid.

Hydrochloric acid is a liquid which is obtained by dissolving hydrogenchloride (a gas with chemical formula HCl) in water. At the most, 37%(to 38%) by weight of HCl gas molecules may be dissolved in water. Forthis maximum concentration, one refers to “pure” hydrochloric acid. Itis this liquid which is used in the present invention for theelectrochemical deposition.

It should be noted that in the prior art (see for example French PatentFR 1 143 213 A) a solution of hydrochloric acid with a concentrationbetween 0% and 20% is used. This means that to obtain such a solution,the pure acid has been diluted and the solution's concentration isbetween 0% of 37% of HCl gas and 20% of 37% of HCl gas.

1. A method for manufacturing at least one electrode on a II-VIsemiconducting material or a compound of this material, this electrodebeing in a metal for which the work function is substantially equal toor larger than that of the II-VI semiconductor, this method beingcharacterized in that the electrode is formed by electrochemicaldeposition of the metal from a solution of a chloride of the metal inpure hydrochloric acid.
 2. The method according to claim 1, wherein themetal is gold or platinum and a gold or platinum chloride solution inpure hydrochloric acid is used.
 3. The method according to claim 2,wherein the concentration of gold or platinum chloride in purehydrochloric acid is less than 5%.
 4. The method according to claim 1,wherein the surface of the material is prepared before the deposition inorder to make this surface capable of fixing the metal.
 5. The methodaccording to claim 4, wherein the surface of the material is chemicallyetched.
 6. The method according to claim 5, wherein the metal is gold orplatinum, a gold or platinum chloride solution in pure hydrochloric acidis used and a solution of bromine and preferably pure hydrochloric acidis used for the chemical etching.
 7. The method according to claim 1,wherein the material is CdTe.
 8. The method according to claim 7,wherein the electrode is formed on a material which is selected fromCdZnTe, CdTe:Cl, CdTeSe:Cl, CdZnTe:Cl, CdTe:In, CdZnTe:In and CdHgTe.